1. Field of the Invention
This invention generally relates to avalanche photodiodes with an impact-ionization-engineered multiplication region. Certain embodiments relate to avalanche photodiodes having higher impact ionization in a first portion of the multiplication region and lower impact ionization in a second portion of the multiplication layer.
2. Description of the Related Art
Light has been used for communication since signal fires were first used to send messages thousands years ago. In the 1960's and 1970's, the invention of the laser and the development of the low-loss optical fiber have ushered in a new era of optical communication. In the past 20 years, fiber optic communication systems have become the dominant backbone of the information-carrying infrastructure across the world due, at least in part, to their high capacity, high speed, relatively low cost, and security. The booming communication market, especially Internet-related systems, has imposed a serious challenge to the capacity of current systems.
Optical communication systems may include a transmitter, a receiver, and a transmission medium to convey energy from the transmitter to the receiver. A laser may be located at the transmitter end of the system and may be modulated with an input electric signal. The output beam may be coupled to an optical fiber through which the optical signal may propagate to the receiver. The receiver may be a photodetector, which may convert the optical signal to an electric signal. The photodetector may be an essential component of the receiver in a fiber optic communication system. In the future, development of relatively high-speed, low-noise, sensitive and low-cost optoelectronic devices may be key to meet future capacity requirements.
An appropriate photodetector for high-bit-rate, long-haul fiber optic communications may be an avalanche photodiode (“APD”). For example, an APD may have an internal gain that provides higher sensitivity than PIN photodiodes. It is important to achieve high sensitivity to maximize the separation between optical repeaters and, thus, reduce the overall system cost. APDs may achieve about 5 dB to about 10 dB better sensitivity than PINs, provided that the multiplication noise is low and the gain-bandwidth product of the APD is sufficiently high. The multiplication region of an APD may play a role in determining the gain, the multiplication noise, and the gain-bandwidth product.